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Re: Science Fair project

12/5/2000 1:03 AM
Re: Science Fair project
I just got off the phone with my amp tech (Steve Wilson) and he was very helpful. I was thinking about changing the phase inverter coupling cap, but I am now going to test the cathode bypass caps. He said to get a rotary switch and have one setting stock, one with no caps (thats ok on the amp, right) and one with a much higher value of caps. He said I could easily test this by making a dummy load with a 4 ohm resistor (I'm using a Bassman) and testing AC voltage across it and how it goes up and down with no caps, stock value caps, and higher value caps (how high up should I go, though?) One problem I have though is that I can't find 4 ohm resistors. Anyone know where I can find one and what wattage it should be. Thanks
12/5/2000 3:17 AM

I assume your tech is talking about using a sine wave to feed your amp. That will give you information about the gain at the frequency of the sinewave.  
Another thing you can try is to feed a bunch of frequencies in at once and see what the tone controls do.  
A square wave consists of the fundamental and every odd order harmonic above it, all added together.  
You might try a 100Hz and a 1000hz square wave through the amp at a relatively low level, like a volt or two out, peak to peak, into a resistor of the same ohms as the amp is rated at, and for only a volt or two, a 25 watt should be fine. Do you have access to a frequency generator and an oscope at schoool or can your science teacher get you access to one, like at a local college or business?  
If you check the effect of the tone controls, you will see there is a definite change in the square wave, meaning a frequency area is beeing eaten or boosted. If you do this and want to know how to read the effect, I'll tell you, but I bet you can tell me after trying it.  
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12/5/2000 3:47 AM

My tech suggested using a radio set in an "in-between" station, basically meaning a position where there is just fuzz and using that as a signal generator. Like I said, I don't know where I can get a 4 ohm resistor. Does anyone have any suggestions.
12/5/2000 5:16 AM

Radio Shack has resistors for this purpose. They are 8 ohms 20 watts, two in parallel is 4 ohms at 40 watts. They are non inductive wire wound, and the RS part number is 271-120.  
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